1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
Recently, demands to increase storage capacities of HDDs (Hard Disk Drives) are increasing. As one means of satisfying such demands, a heat-assisted recording method has been proposed. The heat-assisted recording method performs recording with respect to a magnetic recording medium using a magnetic head mounted with a laser light source, by heating the magnetic recording medium by the magnetic head. The heat-assisted recording method can reduce the coercivity of the magnetic recording medium by heating the magnetic recording medium and enable use of a magnetic material having a high crystal magnetic anisotropy constant Ku (hereinafter also referred to as a “high-Ku material”) for a magnetic layer of the magnetic recording medium. For this reason, the magnetic grain size of the magnetic layer can be reduced while maintaining thermal stability, and a surface recording density on the order of 1 Tbits/inch2 can be achieved.
On the other hand, a microwave-assisted recording method has been proposed as the next-generation recording method that can satisfy the demands to increase storage capacities of the HDDs. The microwave-assisted recording method can perform the recording with respect to the magnetic recording medium with a recording magnetic field lower than or equal to the coercivity of the magnetic recording medium, by the assistance of a high-frequency magnetic field generated from the magnetic head. As a result, a magnetization direction of the magnetic layer is inclined with respect to an axis of easy magnetization, in order to record magnetic information by locally switching the magnetization of the magnetic layer.
For this reason, similarly as in the case of the heat-assisted recording method, the microwave-assisted recording method can use a high-Ku material for the magnetic layer of the magnetic recording medium. The high-Ku magnetic material may be an alloy having an L10 crystal structure.
Ordered alloys, such as L10 type FePt alloys, L10 type CoPt alloys, L11 type CoPt alloys, or the like, have been proposed for the high-Ku material. In addition, in order to separate crystal grains of the ordered alloy, the magnetic layer may be added with a grain boundary material, such as an oxide including SiO2, TiO2, or the like, or C, BN, or the like. By employing a granular structure in which the magnetic crystal grains are separated at the grain boundary, a high medium SNR (Signal-to-Noise Ratio) can be achieved.
Recently, there are demands to further increase the high recording density of the magnetic recording medium. Hence, studies have been reported regarding underlayer configurations that are optimized to further improve the recording density.
One proposed method further improves the recording density by use of a magnetic recording medium having a high perpendicular magnetic anisotropy, for example. The high perpendicular magnetic anisotropy of the magnetic recording medium can be achieved by use of the L10 type ordered alloy having a good (001) orientation for the magnetic layer. The orientation of the L10 type ordered alloy can be controlled by the underlayer, and thus, the orientation of the magnetic layer can be controlled by use of a suitable underlayer.
For example, Japanese Laid-Open Patent Publication No. 11-353648 proposes forming the L10 type FePt alloy on an underlayer that includes MgO or the like as a main component thereof and is controlled to have a (100) crystal plane parallel to the substrate surface, in order to obtain a good (001) orientation of the L10 type FePt alloy.
In addition, as another method of further improving the recording density, there is a method that reduces the grain diameter of the magnetic grains of the magnetic layer. In this case, the alloy having the L10 crystal structure is required to maintain thermal stability even when the magnetic grain diameter of the magnetic layer is reduced.
For example, International Publication No. 2011/021652 proposes adding B, Si, or C to the underlayer made of an alloy that has a BCC (Body-Centered Cubic) structure and includes Cr as a main component thereof and also includes Ti or the like, in order to reduce a magnetic coupling of the magnetic layer and to reduce a cluster size.
However, when the underlayer proposed in Japanese Laid-Open Patent Publication No. 11-353648 or International Publication No. 2011/021652 is used, peeling caused by peel stress between the underlayers, or between the underlayer and the magnetic layer, may be generated, and an anticorrosion property of the magnetic recording medium may be insufficient to thereby deteriorate a reliability of the magnetic recording medium.